Fix the weird bug when username does not contain letters

ssoauth/auth_utils.py

@@ -12,7 +12,7 @@ import importlib
 
 def _validate_username(username):
     assert isinstance(username, str)
-    assert username.islower()
+    assert username == username.lower()  # because .islower() returns False when no letters present
 
 
 def get_user(uuid=None, username=None):